The invention relates to the field of ratchet free wheels allowing unidirectional transmission of torque between two elements. Such free wheels are used, amongst other things, in drivelines for automobiles comprising automatic transmissions and in the hubs of the driven wheels of bicycles.
A ratchet free wheel generally comprises two concentric plates placed face to face. A first plate is equipped on one of its flat faces with a circular path equipped with recesses each formed with a ramp and with an abutment surface arranged at the opposite end of the ramp. Ratchets are supported by the second plate and collaborate with the recesses of the first plate.
When the plates are given a relative angular displacement in one direction about the axis of rotation, the ratchets slide with respect to the ramps and thus manage to leave a recess to pass from one recess to another without transmitting torque. When the relative movement is in the opposite direction, at least one ratchet collaborates with an abutment surface of a recess of the first plate. The two plates are therefore connected in terms of rotation and can transmit torque between them; please see documents EP-A-0 522 983 and FR-A-2 188 551.
This then yields a system of unidirectional connection between two elements capable of transmitting torque between a driving element and a driven element, in just one direction.
The present invention sets out to improve ratchet free wheels.
More particularly, the present invention proposes to provide a ratchet free wheel with a disengagement means.
What happens is that in certain applications, such as gear or sprocket couplings, it may be advantageous to have a disengagable system that can operate either as a unidirectional connection in terms of rotation between two elements or as a system the two elements of which are completely independent of one another in terms of rotation and can transmit no torque regardless of their relative direction of rotation.
The ratchet free wheel device according to one aspect of the invention comprises a first element provided with uniformly distributed cavities, a number of ratchets able to project into said cavities, and a second element for supporting the ratchets so that the second element rotates as one with the first element in one direction of relative rotation of said first and second elements and is free to rotate in an opposite direction of relative rotation. Each ratchet comprises a head able to project into said cavities and a shank collaborating with said control means. The device comprises a means of controlling the ratchets which is capable of preventing them from projecting into said cavities so that the second element is free to rotate with respect to the first element in two opposite directions of relative rotation of said first and second elements when the control means is active.
Benefit is derived from the possibility of disengaging the free wheel device by activating the control means. This then switches from a unidirectional connection to a free two-directional movement.
As a preference, the device comprises an operating element able to collaborate with the ratchets so as, in an inactive position, to leave them free to project into said cavities and, in an active position, to keep them out of said cavities.
The operating element may be in the form of an annular component comprising a radial flange pierced with holes through which the ratchets can pass, and a radial rim designed to collaborate with a fork. The rim and the flange may be connected by a tubular portion.
In one embodiment of the invention, the device comprises a control system collaborating with the operating element and determining whether it is in the inactive or in the active position.
The head advantageously has a diameter greater than that of the shank. The head and the shank may be coaxial.
Advantageously, the shank of each ratchet projects from the second element away from said cavities and is equipped with a retaining means capable of collaborating with said control means. The retaining means will be unidirectional in terms of translation. In other words, the control means may, by virtue of the retaining means, exert on the shank of a ratchet a force directed away from the head of the ratchet and will be unable to exert on the shank of a ratchet a force directed toward the head of the ratchet.
In one embodiment of the invention, the retaining means comprise a washer fixed to the shank of a ratchet, so that a portion of the control means is arranged between the second element and the washer. The washer may be of the frustoconical type, forcibly fitted onto the shank of a ratchet in order to provide axial retention.
In one embodiment of the invention, each ratchet has an axisymmetric shape, the head being cylindrical, and the shank also being cylindrical. Such a ratchet is very economical to manufacture.
In one embodiment of the invention, each ratchet comprises a radial bearing surface between the head and the shank, a spring being arranged around the shank and bearing against said bearing surface.
In one embodiment of the invention, the spring also bears against the second element and tends to cause each ratchet to project into said cavities.
More specifically, the first element is of annular shape with an L-shaped cross section with a radial portion and an axial portion. A circular path with recesses is formed on the lateral face of the radial portion arranged on the same side as the axial portion. Each recess has a flat bottom delimited on one side in the circumferential direction by an inclined ramp and on the opposite side by an abutment surface directed axially.
The second element also has an annular shape of L-shaped cross section with a radial portion and an axial portion and is mounted concentrically with respect to the first element. The radial portions of the two elements are arranged facing one another via one of their faces. The second element has a certain number of drillings or cells intended to accommodate the axially mobile ratchets.
The ratchets have the form of a cylindrical piston and are arranged in the drillings of the second element while being thrust axially by a return spring toward the first element to collaborate with the recesses of the first element via one of their ends known as the head of the ratchet.
The second element is centered on the first element by means of a cylindrical bearing surface arranged in its bore and collaborating with a cylindrical bearing surface formed on the periphery of the axial portion of the first element. The axial portion of the second element protrudes slightly with respect to the large lateral face of the first element, thus allowing a circlip to be fitted in a groove formed in the free end of the axial portion of the second element and thus allowing the two elements to be secured together axially.
The ratchet free wheel device thus formed is mounted, on the one hand, on a shaft via the first element and, on the other hand, in a housing via the second element. The angular connection between the housing and the free wheel device is provided by its splines. The connection between the shaft and the free wheel device may also be by means of splines.
In one direction of relative rotation of shaft and housing, the ratchets can move up the ramps of the recesses and sink axially into the cells to then drop into the next recess under the thrust of the associated spring, and so on. In the opposite direction of rotation, the ratchets come into contact with the abutment surface of the recesses and cannot pass into the next recess. The two plates are thus angularly connected in this direction and may thus transmit torque via the piston-form ratchets.
Of course, the device will initially be designed so that the number of ratchets simultaneously in engagement with the abutment surfaces is a function of the torque that is to be transmitted. As the torque that can be transmitted with this type of ratchet free wheel is very high, it is often possible for the desired torque to be transmitted with just one ratchet in engagement. It is therefore possible to envisage a number of ratchets other than the number of recesses and for them to be distributed angularly in such a way that the take-up of torque or locking when switching from free wheel operation to torque-transmitting operation occurs in an angle smaller than the angle between two recesses.
By way of example, if the free wheel has ten recesses and ten ratchets simultaneously in torque-transmitting engagement, the angular play or torque take-up angle will be equal to 360xc2x0/10=36xc2x0. If the free wheel has ten uniformly distributed recesses and eleven ratchets, also uniformly distributed, just one ratchet may be in torque take-up engagement. The switch from the free wheel position to the drive position occurs over a maximum angle of 360xc2x0/(10xc3x9711)=3xc2x0 and 16 minutes.